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I have a spot in my detached garage that reeks of some 'mercaptan'y smell. It has for 25 years. There is no natural gas within 1/2 mile. My propane tank is 100feet away on the opposite side of my house- which is 50 feet from the other side of the garage.
I've torn everything out to the inside of the T-1-11 siding- smell is gone for a day, or a week, or a month-- then I smell it again. I'm guessing the previous owner spilled something there-- or it is right in the concrete.
Hmmm--- I did find a prosthetic leg in the attic. . . maybe the old leg is beckoning from below the garage?
Jim